CALCIUM HANDLING BY SARCOPLASMIC-RETICULUM OF NEONATAL SWINE CARDIAC MYOCYTES

In recent years, because of similarities to human infants, neonatal pi glets have increasingly become the model of choice for studying neonat al heart function. However, the cardiac sarcoplasmic reticulum (SR) ha s not been thoroughly characterized in this species. Accordingly, Ca2 pump kinetics, efflux channel characteristics, Ca2+ transients, and c ontractile movements were examined in isolated newborn piglet cardiac ventricular myocytes. Maximum uptake rate (V-max) and concentration re quired to produce a half-maximal effect (K-0.5) for oxalate-supported, ATP-dependent; Ca-45(2+) uptake by the SR of digitonin-lysed myocytes were 285 +/- 17 nmol Ca-45(2+).min(-1).mg(-1) and 0.69 +/- 0.07 mu M, respectively. In the absence of phospholamban phosphorylation, V-max was reduced to 195 +/- 26 nmol Ca-45(2+).min(-1).mg(-1) (P < 0.05 vs. control) and K-0.5 increased to 1.28 +/- 0.13 mu M (P < 0.05 vs. contr ol). [H-3]ryanodine binding studies yielded a maximum binding capacity of 181 +/- 12 fmol/mg and a dissociation constant of 1.7 +/- 0.2 nM. Raising extracellular Ca2+ (0.5-5 mM) increased peak amplitude and dec reased the duration of electrically stimulated fura 2 Ca2+ transients and recordings of cell length changes. Both ryanodine and 2,5-di-tert- butylhydroquinone, an inhibitor of SR calcium adenosinetriphosphatase, completely abolished Ca2+ transients in piglet myocytes. These studie s indicate that the SR has a significant role in excitation-contractio n coupling in neonatal piglet myocytes.